The present invention relates to an axially asymmetrical non-spherical F.theta. lens having a complicated shape and to a method for making same, as well as to a laser beam printer using that lens.
At first, the construction of a laser printer will be explained, referring to drawings. A laser beam printer consists mainly of a scanning optical system (FIG. 21), which modulates-and deflects a laser light beam emitted by a light source 39 so as to form a light pattern on a light sensitive body 43 and an image forming system (FIG. 22), which transforms the light pattern formed on the light sensitive body 43 by the scanning optical system into a hard copy by using an electronic photographic process.
In FIG. 21, generally a gas laser or a semiconductor laser is used for the light source 39. Further, generally an A/O modulator using an acoustic optical (A/O) element is used for the modulator 47. The A/O modulator intensity-modulates an injected laser light beam by diffracting it by synchronous variations in the refractive index generated by an ultrasonic wave, which is made to pass through the A/O element. In order to increase modulation speed by the A/O element, a beam compressor 48a for decreasing the diameter of the injected beam, a beam expander 48b used for obtaining a small image spot on the light sensitive body, and a collimator lens 41 for transforming a diverging beam emitted by the semiconductor laser into a parallel beam are used. Further a rotary polygonal mirror 42 is used for the deflector for scanning the light sensitive body with the laser light beam. In addition, a laser printer using a hologram instead of the rotary polygonal mirror is also proposed.
Since the rotary polygonal mirror 42 is rotated with a constant speed, the reflected laser light beam is deflected with a constant angular velocity. An imaging lens (F.theta.) 38 has a function (f.theta. characteristics) of giving the injected light beam deflected with a constant angular velocity optical distortion for transforming the beam so as to scan the surface of the light sensitive body with a constant speed.
The light sensitive body 43 has a two-layered structure in which a photoconductive layer is disposed on a conductive supporter. The surface of the light sensitive body is previously uniformly charged in the dark by discharge of plus corona 49, etc. (FIG. 22) When this is irradiated with laser light, the resistance of the photoconductor decreases at the part irradiated with light so that electric charge existing there flows to ground, which gives rise to parts where electric charge still exists and parts where there exists no more electric charge on the surface of the light sensitive body 43.
A latent image formed on the light sensitive body 43 is developed by toner charged positively or negatively. As indicated in FIG. 22, the electric charge on the surface of the insulating layer of the light sensitive body 43 is removed by corona discharge 50 and at the same time it is irradiated with laser light through the imaging lens 38. At clear parts irradiated with laser light the resistance of the photoconductive layer decreases so that it becomes conductive and thus electric charge on the front and the rear surface of the insulating layer decreases rapidly. At dark parts, which are not irradiated with laser light, although the surface of the insulating layer is exposed to AC corona discharge 50 and the potential thereof becomes approximately zero, the electric charge formed at the interface between the insulating layer and the photoconductive layer is held.
As described above, after a charged layer has been formed by the primary charging at the interface between the insulating layer and the photoconductive layer, the electric charge on the surface of the insulating layer is removed by the corona charge elimination, and at the same time it is exposed by irradiating it with laser light. Thereafter the whole surface of the light sensitive body 43 is exposed uniformly by a whole surface exposer 51 so that the superficial potential at the dark parts is raised. The latent image formed on the light sensitive body 43 is developed by using toner charged positively or negatively in a developing device 52. After a developing step, the toner image thus obtained on the light sensitive body 43 is transcribed electrostatically on a plain paper sheet sent from a sheet supplying cassette 53 through a sheet supplying roller 54 and transformed into a stable permanent image by a fixing step by means of a fixing device 55. The plain paper sheet, on which the image is transcribed, is sent to a stacker 56. After the transcribing step, toner remaining on the light sensitive body, which has not been transcribed, is removed by a cleaning step by means of a cleaner 57 and a cleaning blade 58. In this way the light sensitive body 57 is prepared for a succeeding latent image forming process. The laser beam printer is described e.g. in "Laser Beam Printer" (Shashin Kogyo or Photography Industry) by Kitamura and Hirayama, February 1976, pp. 89-92.
For such a laser printer following propositions have been done in order to achieve printing of high quality without increasing the number of parts.
For example, in JP-A-2-23313, a construction is disclosed, for which the F.theta. lens has an asymmetrical non-spherical shape, in which the radius of curvature in the direction of tilt of facets (in the auxiliary scanning direction) thereof increases with increasing distance from the optical axis.
Another method for fabricating such an asymmetrical non-spherical lens is disclosed in JP-A-2-533557. This is a method, by which the lens is fabricated by grinding under NC control. By this method it is possible to generate an arbitrary non-spherical shape. However, by this method, a long working time is necessary (7 pieces for 3 hours) and therefore there was a problem in the mass productivity.
Still another method has been proposed as a method of fabricating a non-spherical lens, which is excellent in the mass productivity, by which a plastic lens excellent in the shape precision and the surface precision is obtained in addition to the molding method using a plastic substance as raw material. (refer to e.g. JP-A-59-204001)
However, concerning a female die necessary for molding processing, almost all the optical surfaces available at present are plane or spherical. Further, even if they are non-spherical, they are axially symmetrical non-spherical surfaces such as those used in a single lens reflex camera and it is not possible to fabricate axially asymmetrical non-spherical concave surfaces. That is, although it is possible to fabricate symmetrical non-spherical surfaces by molding processing, at the present state it is not possible to fabricate axially asymmetrical non-spherical surfaces in mass production by molding processing. Furthermore, in plastic lenses, deformations are easily produced by variations in the temperature at use, which can cause aberrations and deviations in the optical axis. Still further the plastic lenses are apt to be influenced by humidity and chemical substances in the atmosphere. In order to solve this point, there are proposed a method for forming a protecting film on the lens surface (e.g. JP-A-55-28023) and a lens protecting film forming method, by which an organic silicon protecting film is formed by evaporation (e.g. JP-A-56-25701). However, by these methods, since it is difficult to control the protecting film thickness and a process of baking the evaporated film is necessary, it is not possible to fabricate axially asymmetrical non-spherical lenses of high quality and mass-productivity. Therefore it is an important problem to be solved to develop a technique for generating a concave surface having an axially asymmetrical non-spherical shape to obtain an axially asymmetrical non-spherical female die with a high precision.